If you want to see detail explain, you can read the article How To Download macOS Mojave Installer And Convert To ISO File.

You can follow below brief steps to do it.

Now it is time to convert the above macOS high sierra installer to an iso file to install it in VirtualBox.

Create ISO File From macOS High Sierra Installer. drwxr-xr-x 11 root wheel 352 Dec 14 19:03 ContentsĢ. drwxr-xr-x 11 root wheel 352 Dec 14 19:03 Contents

You can also find it in the terminal using the below command.

When the download completes, the macOS High Sierra installer file will be saved in the /Applications folder, the installer file name is Install macOS High Sierra.

During the download process, you can see the remaining time of the download.

Click the blue GET button to popup the download macOS High Sierra prompt dialog, it will ask you whether to download it now or later, click the Download button to start the download.

And it will show the macOS High Sierra download page. Click it will open the App Store in your current macOS ( my current os is Mojave ).

After searching the sentence macOS High Sierra download in google, I finally find the macOS high sierra download link.

So I need to download the macOS High Sierra.

At first, I download the newest macOS Mojave following the article How To Download macOS Mojave Installer And Convert To ISO File, and when I have done creating the iso file, I find VirtualBox does not support macOS Mojave, the highest version that Virtualbox supports is macOS High Sierra.

Install macos high sierra virtualbox